Photoelectron Velocity Research Articles

Multicenter electron-sharing σ-bonding in the AgFe(CO)4- complex.

The heterodinuclear silver tetracarbonyl-iron anion was generated in the gas phase and studied by joint photoelectron velocity map imaging spectroscopy and quantum chemical calculations. The AgFe(CO)4- anion is characterized to be an 18-electron complex with the silver atom covalently bonded to the anionic tetracarbonyl-iron, an isolobal analogue of the methyl radical. The bonding analyses using a range of state-of-the-art quantum chemistry methods revealed a peculiar decentralized bonding situation, where the silver atom is covalently bonded to both the iron center and the vicinal carbon atoms in the form of an electron-sharing σ bond.

Characterization of the Ground States of BeC2 and BeC2- via Photoelectron Velocity Map Imaging Spectroscopy.

Due to their potentially unique properties, beryllium carbide materials have been the subject of many theoretical studies. However, experimental validation has been lacking due to the difficulties of working with Be. Neutral beryllium dicarbide has been predicted to have a T-shaped equilibrium structure (C2v), while previous quantum chemistry calculations for the structure of the anion had not yielded consistent results. In this study, we report photoelectron velocity map imaging spectra for the BeC2- X 2A1 → BeC2 X 1A1 transition. These data provide vibrational frequencies and the electron affinity of BeC2. Ab initio electronic structure calculations, validated against the experimental data, show that both the anion and the neutral form have C2v equilibrium geometries with polar covalent bonding between Be and the C2 subunit. Computed vibrational frequencies and the electron affinity, obtained at the CCSD(T) level of theory, were found to be in good agreement with the measurements.

Relaxation Dynamics and Genuine Properties of the Solvated Electron in Neutral Water Clusters.

We have investigated the solvation dynamics and the genuine binding energy and photoemission anisotropy of the solvated electron in neutral water clusters with a combination of time-resolved photoelectron velocity map imaging and electron scattering simulations. The dynamics was probed with a UV probe pulse following above-band-gap excitation by an EUV pump pulse. The solvation dynamics is completed within about 2 ps. Only a single band is observed in the spectra, with no indication for isomers with distinct binding energies. Data analysis with an electron scattering model reveals a genuine binding energy in the range of 3.55–3.85 eV and a genuine anisotropy parameter in the range of 0.51–0.66 for the ground-state hydrated electron. All of these observations coincide with those for liquid bulk, which is rather unexpected for an average cluster size of 300 molecules.

Correction to “Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging Studies of Thin Au Film and Single Supported Au Nanoshells”

Correction to “Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging Studies of Thin Au Film and Single Supported Au Nanoshells”

Photoelectron imaging spectroscopic study and chemical-bonding analysis of AgOH and AgSH anions

The photodetachment spectra of AgOH− and AgSH− have been investigated using photoelectron velocity map imaging (PE-VMI). In combination with the ab intio calculations, vibration-resolved photoelectron spectra yield accurate electron affinities of 1.177(15) eV and 1.193(10) eV for AgOH and AgSH, respectively. Franck-Condon simulations of the ground state transition are performed to assign vibrational structure in the spectra and to assist us in identifying the observed spectral bands. And the frequencies are extracted to be 475(35) cm−1 for AgOH and 358(30) cm−1 for AgSH, respectively. A variety of theoretical calculations are analyzed to reveal the chemical bonding characteristics of AgOH−1/0 and AgSH−1/0, which indicates that Ag-X bonding in AgXH−1/0 (X = O and S) species is much more ionic than covalent.

Photoelectron imaging of resonance-enhanced multiphoton ionization and above-threshold ionization of ammonia molecules in a strong 800-nm laser pulse**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574116, 11534004, 10704028, and 11474123).

In this work, we mainly investigate the NH3 molecular multiphoton ionization process by using the photoelectron velocity map imaging technique. Under the condition of femtosecond laser (wavelength at 800 nm), the photoelectron images are detected. The channel switching and above-threshold ionization (ATI) effect are also confirmed. The kinetic energy spectrum (KES) and the photoelectron angular distributions (PADs) are obtained through the anti-Abel transformation from the original images, and then three ionization channels are confirmed successfully according to the Freeman resonance effect in a relatively low laser intensity region. In the excitation process, the intermediate resonance Rydberg states are (6 + 2 photons process), (6 + 2 photons process) and (7 + 2 photons process), respectively. At the same time, we also find that the photoelectron angular distributions are independent of laser intensity. In addition, the electrons produced by different processes interfere with each other and they can produce a spider-like structure. We also find ac-Stark movement according to the Stark-shift-induced resonance effect when the laser intensity is relatively high.

Charging and dynamics of dust particles in lunar photoelectron sheath

Sunlight scattering from electrostatically charged floating particles is considered accountable for the lunar twilight observations of horizon glow and streamers. In this work, the dynamics of the fine charged particles within the photoelectron sheath over the sunlit lunar surface has been investigated. Accounting for the influence of solar radiation, solar wind plasma, and lunar gravity, the present transport model consistently takes account of the coexisting phenomenon of particle charging, and characteristic photoelectron sheath screening in deriving the vertical motion of the particle. As novel features, half Fermi-Dirac statistics of the photoelectron velocity in determining the electron population and the sheath structure through the Poisson equation, and anisotropic photoelectron flux in evaluating the particle charge as it traverses through the sheath, have adequately been included in the analysis. In this framework, the electrostatic sheath features are observed to dominate over the lunar gravity in determining the dynamics of smaller (nanometer) size grains; for instance, ∼10 nm sized particles detached from the lunar regolith with a finite velocity are shown to loft up to the kilometer altitude. Depending on the initial particle velocity and size, a parametric regime defining the particle hopping over the lunar surface has been identified.

Magic Numbers for the Photoelectron Anisotropy in Li-Doped Dimethyl Ether Clusters.

Photoelectron velocity map imaging of Li(CH3OCH3)n clusters (1 ≤ n ≤ 175) is used to search for magic numbers related to the photoelectron anisotropy. Comparison with density functional calculations reveals magic numbers at n = 4, 5, and 6, resulting from the symmetric charge distribution with high s-character of the highest occupied molecular orbital. Since each of these three cluster sizes correspond to the completion of a first coordination shell, they can be considered as “isomeric motifs of the first coordination shell”. Differences in the photoelectron anisotropy, the vertical ionization energies and the enthalpies of vaporization between Li(CH3OCH3)n and Na(CH3OCH3)n can be rationalized in terms of differences in their solvation shells, atomic ionization energies, polarizabilities, metal–oxygen bonds, ligand–ligand interactions and by cooperative effects.

Spectroscopic identification of the low-lying electronic states of S2 molecule.

As is well-known, the S2 molecule is a ubiquitous intermediate in the combustion, atmosphere, and interstellar space. The six low-lying bound states of S2 have been characterized via photoelectron velocity map imaging and a high-level multi-reference configuration interaction method with the Davidson correction. Spectroscopic constants have been extracted by fitting the potential energy curves extrapolated to the complete basis set limit with a series of Dunning's correlation-consistent basis sets: aug-cc-pV(Q, 5)Z. The calculated spectroscopic parameters well reproduce the experimental results in this work. On the basis of the theoretical calculations, Franck-Condon simulations are performed to assign six adjacent electronic states, especially for three higher overlapping electronic states (c1Σu -, A'3Δu, and A3Σu +). The dissociation energy De of the S2 - is evaluated to be 4.111 (4) eV in this work, in agreement with the theoretical prediction (4.056 eV).

High-Frequency Quasi-Potential Waves in the Plasma Formed under Tunnel Ionization of Atoms

The dispersion law and collisionless damping rate of quasi-potential waves in the plasma formed upon tunnel ionization of gas atoms in the field of a short pulse of circularly or linearly polarized radiation are found. It is shown how the frequency and damping rate of quasi-potential waves depend on the wave propagation direction relative to the symmetry axis of the photoelectron distribution. It is established that, in plasma with a toroidal photoelectron velocity distribution, weakly damped waves with a linear dispersion law and frequency above the electron plasma frequency can propagate in a wide range of angles. In the case of a bi-Maxwellian photoelectron distribution, the frequency of weakly damped waves is comparable with the electron plasma frequency and the anisotropy of electron motion manifests itself in relatively small corrections to the dispersion law.

Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging Studies of Thin Au Film and Single Supported Au Nanoshells

Transverse (2D) photoelectron velocity distributions are directly measured on 10 nm Au film and single Au nanoshells following multiphoton excitation/photoemission. This unique capability is achieved by combining scanning photoemission microscopy with velocity map imaging, yielding photoelectron spectra as a function of diffraction-limited position on a sample. Detailed 3D photoelectron velocity distributions are retrieved for Au film by fitting the 2D data with a ballistic (three-step) photoemission model, where contributions from two-photon, three-photon, and d-band processes are identified and further characterized as a function of photon energy using a broadly tunable, visible femtosecond optical parametric oscillator. These techniques are further applied to investigate the more complex behaviors of single plasmonic Au nanoshells with silica cores. The strong plasmonic near- and far-field signatures are first characterized via optical and photoemission measurements, along with theoretical methods (Mie...

Vibronic relaxation dynamics of o-dichlorobenzene in its lowest excited singlet state

Vibronic dynamics of o-dichlorobenzene in its lowest excited singlet state, S1, is investigated in real time by using femtosecond pump-probe method, combined with time-of-flight mass spectroscopy and photoelectron velocity mapping technique. Relaxation processes for the excitation in the range of 276–252 nm can be fitted by single exponential decay model, while in the case of wavelength shorter than 252 nm two-exponential decay model must be adopted for simulating transient profiles. Lifetime constants of the vibrationally excited S1 states change from 651 ± 10 ps for 276 nm excitation to 61 ± 1 ps for 242 nm excitation. Both the internal conversion from the S1 to the highly vibrationally excited ground state S0 and the intersystem crossing from the S1 to the triplet state are supposed to play important roles in de-excitation processes. Exponential fitting of the de-excitation rates on the excitation energy implies such de-excitation process starts from the highly vibrationally excited S0 state, which is validated, by probing the relaxation following photoexcitation at 281 nm, below the S1 origin. Time-dependent photoelectron kinetic energy distributions have been obtained experimentally. As the excitation wavelength changes from 276 nm to 242 nm, different cationic vibronic vibrations can be populated, determined by the Franck-Condon factors between the large geometry distorted excited singlet states and final cationic states.

Photoelectron Velocity Map Imaging Spectroscopy of Heteronuclear Metal–Nickel Carbonyls MNi(CO) n − (M = Sc, Y; n = 2–6)

The chemical bonding and electronic structure of heteronuclear metal–nickel carbonyls MNi(CO) n − (M = Sc, Y; n = 2–6) have been investigated by mass-selected photoelectron velocity map imaging spectroscopy and quantum chemical calculations. Two CO bonding modes (side-on-bonded and terminal carbonyls) are involved in the n = 2 cluster. The building block composed of three kinds of different CO modes (side-on-bonded, bridging, and terminal carbonyls) is favored at n = 3, the structure of which persists up to n = 6. The additional CO ligands are preferentially coordinated in the terminal mode to the Sc atom and then to the Ni atom in the larger clusters. The present findings would promote the understanding of CO molecule activation and chemisorbed CO molecules on metal surfaces.

An approach for improving signal-to-noise ratio in photoelectron velocity map imaging experiments

Background irradiated electrons in photoelectron spectra are known to take place under the ultraviolet light irradiation. Therefore, it deserves discussing the reduction of such electrons in the electron spectra in this work. The main problem present in these electron spectra will largely be overcome through a workflow and process, namely, a combination method of rotating images with angular-tailored and subtracting noise electrons of images which gives rise to non-noise electron peaks. An improved SNR is readily obtained for REMPI-SEVI spectra of 3-bromofluorobenzene (3-Br-FPh) compared with no noise–subtracted and no-angular-tailored spectra. More details are given including electron velocity-map imaging analysis on S−, CuOH−(H2O), and 3-Br-FPh in the following sections.

Influence of long-range Coulomb interaction in velocity map imaging.

The standard velocity-map imaging (VMI) analysis relies on the simple approximation that the residual Coulomb field experienced by the photoelectron ejected from a neutral or ion system may be neglected. Under this almost universal approximation, the photoelectrons follow ballistic (parabolic) trajectories in the externally applied electric field, and the recorded image may be considered as a 2D projection of the initial photoelectron velocity distribution. There are, however, several circumstances where this approximation is not justified and the influence of long-range forces must absolutely be taken into account for the interpretation and analysis of the recorded images. The aim of this paper is to illustrate this influence by discussing two different situations involving isolated atoms or molecules where the analysis of experimental images cannot be performed without considering long-range Coulomb interactions. The first situation occurs when slow (meV) photoelectrons are photoionized from a neutral system and strongly interact with the attractive Coulomb potential of the residual ion. The result of this interaction is the formation of a more complex structure in the image, as well as the appearance of an intense glory at the center of the image. The second situation, observed also at low energy, occurs in the photodetachment from a multiply charged anion and it is characterized by the presence of a long-range repulsive potential. Then, while the standard VMI approximation is still valid, the very specific features exhibited by the recorded images can be explained only by taking into consideration tunnel detachment through the repulsive Coulomb barrier.

Photoelectron Velocity Map Imaging Spectroscopy of Lead Tetracarbonyl-Iron Anion PbFe(CO)4(.).

Joint research of photoelectron velocity map imaging spectroscopy and density functional theory has been performed to probe the geometrical structures and electronic properties for heterodinuclear iron-lead carbonyl cluster PbFe(CO)4(-), which serves as a monomer of the metal-metal bonded oligomer. The photoelectron detachment of PbFe(CO)4(-) is recorded at two different photon energies with rich spectral features. The ground-state transition obtained at 532 nm reveals a broad vibrationally resolved spectral band, which corresponds to the lead-iron stretching, while the 355 nm spectrum displays many more transitions on the higher-energy side, which correspond to the electronic excited states of PbFe(CO)4. Theoretical calculations at the B3LYP level are performed to explore the ground states of both the anionic and neutral PbFe(CO)4 and to support spectral identification of the fine resolved photoelectron spectra. Moreover, the unique chemical bonding between lead and iron in PbFe(CO)4 is discussed with the aid of natural bond orbital analyses.

Calculation of photodetachment cross sections and photoelectron angular distributions of negative ions using density functional theory.

Recently, the development of photoelectron velocity map imaging makes it much easier to obtain the photoelectron angular distributions (PADs) experimentally. However, explanations of PADs are only qualitative in most cases, and very limited works have been reported on how to calculate PAD of anions. In the present work, we report a method using the density-functional-theory Kohn-Sham orbitals to calculate the photodetachment cross sections and the anisotropy parameter β. The spherical average over all random molecular orientation is calculated analytically. A program which can handle both the Gaussian type orbital and the Slater type orbital has been coded. The testing calculations on Li(-), C(-), O(-), F(-), CH(-), OH(-), NH2 (-), O2 (-), and S2 (-) show that our method is an efficient way to calculate the photodetachment cross section and anisotropy parameter β for anions, thus promising for large systems.

Ultrafast dynamics of ethylbenzene cations probed by photofragmentation and photoelectron spectrometry

Ultrafast dynamics of ethylbenzene (EB) cation has been studied by time-resolved photofragmentation mass-spectrometry (PFMS) in combination with photoelectron spectrometry. EB cations were prepared by multiphoton ionization. The photoelectron spectrum was registered by photoelectron velocity map imaging (PEVMI) and the observed bands were assigned to different cation states. The ultrafast dynamics of these cation states are measured by monitoring the time dependent parent-ion depletion and the fragment-ion appearance, simultaneously. An ultrafast relaxation time of the parent ion of 2.50 (±0.13) ps has been inferred, which is attributed to a dissipative intramolecular vibrational redistribution (IVR) process within the D0 cation state. The results were compared with o-xylene cations, an isomer of EB.

Photoelectron holography in strong optical and dc electric fields

The application of velocity map imaging for the detection of photoelectrons resulting from atomic or molecular ionization allows the observation of interferometric, and in some cases holographic structures that contain detailed information on the target from which the photoelecrons are extracted. In this contribution we present three recent examples of the use of photoelectron velocity map imaging in experiments where atoms are exposed to strong optical and dc electric fields. We discuss (i) observations of the nodal structure of Stark states of hydrogen measured in a dc electric field, (ii) mid-infrared strong-field ionization of metastable Xe atoms and (iii) the reconstruction of helium electronic wavepackets in an attosecond pump-probe experiment. In each case, the interference between direct and indirect electron pathways, reminiscent of the reference and signal waves in holography, is seen to play an important role.

Vibrationally Resolved Photoelectron Imaging of Au3H–

We report a combined photoelectron velocity map imaging spectroscopy and density functional theory investigation on the Au3H(-) anion. Transition between the anionic electronic ground state and the neutral electronic ground state is revealed. Vibrationally resolved spectra were recorded at two different photon energies, providing a wealth of spectroscopic information for the electronic ground state of the Au3H. Franck-Condon simulations of the ground-state transition are carried out to assist in the assignment of the vibrationally resolved spectra. The electron affinity and vertical detachment energy of Au3H are measured to be 2.548 ± 0.001 and 2.570 ± 0.001 eV, respectively. Three stretching vibrational modes are determined to be activated upon photodetachment, with the frequencies of 2100 ± 100, 177 ± 10, and 96 ± 10 cm(-1).